1. The Field of the Invention
This invention relates to composite materials and, more particularly, to novel systems and methods for improving resin flow into reinforcing preforms of composite structures, as well as improving structural performance of completed composite, reinforced structures.
2. The Background Art
Fiber-reinforced, composite materials typically include a resinous matrix reinforced with fibers embedded in the matrix. Fibers may be formed in individual rovings or tows. Reinforcing fibers may be oriented parallel to one another or across one another. In many instances, intermediate layers of non-oriented mats may also be formed of reinforcing fibers. Various reinforcing materials may include glass, kevlar(trademark), boron, carbon (graphite), and the like. Typical resins are thermoset polymers such as epoxy, polyester, and polyamides.
Composite materials may be formed in sheets to be flexible and somewhat deformable thus, composite materials may be designed as partially-cured, resin-loaded panels, tubes, and the like. In other embodiments, composite materials may be formed as wound structures with resins flowing onto the structure with the rovings, as the rovings are wound thereon. In other embodiments, preforms may contain a large fraction of their eventual resin content, partially cured or uncured but highly viscous or even thixotropic. Thus, a preform may appear in the approximate or exact shape of a structure, although not cured. Typically, during cure, heat facilitate chemical reactions converting liquid resins to solids, or stiff, flexible resins to solids, bonding the reinforcing fibers with the resins into a rigid solid.
In conventional curing, parts may be contained in an autoclave at pressures and temperatures elevated with respect to ambient conditions. Typically, pressures in autoclaves are applied to prevent entrained or absorbed air and other non-condensible (or condensible) gases from expanding, bubbling, and therefore weakening, altering, shifting, or otherwise damaging the integrity, strength, or shape of a structure.
Some preforms are designed to receive a substantial quantity of resin beyond the amount with which they were originally formed. Accordingly, structures may be laid into molds or placed in vacuum bags to facilitate flow of additional resin into the interstitial spaces remaining within the rovings, or even the matrix, of a preform.
Limitations arise in manufacturing composite materials by conventional methods. At least three difficulties arise with molded preforms. First, preforms contain sufficient resin to greatly inhibit the flow of additional resin through the rovings, and especially past the matrix portions of a preform. Second, conventional molding techniques flow resin through a comparatively circuitous path of a preform. Thus, flows are uneven, subject to the boundary layer effects well known in fluid mechanics and rheology, and the subsequent variations in pressure, void percentage, gas entrainment, gas absorption, and the like.
Moreover, since preforms typically have a substantial fraction of their ultimate resin content already in a stabilized, suspended, partially cured, or otherwise committed state, bubbles are virtually impossible to prevent or remove in the layup matrix. Even if a matrix is not a complete xe2x80x9cPrePregxe2x80x9d system having all of the resin present, obtaining adequate, well-distributed, gasless flows of resins is difficult.
Moreover, the xe2x80x9cplumbingxe2x80x9d required for fluid handling can be enormous. For example, the air flows accommodated are only the beginning of problems. Flows of matrix resins must be accommodated within the preform, the eventual part to be cast, but likewise through breather mats, runners, and various other flow lines designed to carry the matrix resin to the structure molded, and away therefrom. Cleanup and reuse of runners, tables, and the like requires substantial effort for timely removal of excess resin flows.
Third, the question of vacuum versus pressure tends to provide uneasy compromises. Molding liquid resins under vacuum conditions can remove bubbles. However, compression molding can provide closer proximity between rovings, improving strength. Pressurizing a composite layup tends to leave absorbed or entrained gases. Vacuum drawn on a preform or composite layup during manufacture tends to release gases from solution or entrapment creating bubbles that damage structural integrity.
Currently, pressurized autoclaves, and vacuum chambers both fail to completely satisfy the need to remove trapped or entrained air, non-condensible gases and the like, while also providing structural proximity for roving fibers throughout the structure, as needed for maximum strength.
What is needed is an apparatus and method adapted to create preforms in a manner that will maintain structural shapes and dimensions precisely, while still accommodating high speed flows of resin therethrough, completely sweeping the matrix space. Likewise, ready evacuation of all gases from a preform, prior to flooding with resin, is needed.
What is also needed is an improved resin flow path that does not require the current, clumsy, cluttered, collection of pipes, pumps, and passages for transferring gases and liquids. A clean, straightforward method for resin infusion is needed.
Also what is needed is a process that can take place outside an autoclave and thus accommodate larger parts, while eliminating bubbles by optimizing the vacuum history of a mold. Doing so would be very beneficial if done while also maximizing structural integrity by providing a vise component or clamping component to place tows, rovings, or the like in preforms, mats, or other shapes under the proper loads to assure the best structural properties. Thus, this last requirement requires a new balance using vacuum techniques, pressure techniques, and combinations to obtain the best performance of each.
In view of the foregoing, it is a primary object of the present invention to provide three new processes, and apparatus for accomplishing those processes, as well as a device or apparatus as an output of one or more of those processes.
It is an object of the invention to provide a resin-stitching process by which a preform of virtually any shape may be constructed having sufficient resin to maintain structural shape and integrity during handling and molding processes but with sufficient space to eventually add, flow, and distribute the majority of resin to the preform.
It is an object of the invention to provide a resin-packet-transfer process by which resin is applied to a preform to flow transversely through the minimum dimension of a structure or layup of rovings, rather than flowing only longitudinally along reinforcing fibers over comparatively extensive distances.
It is an object of the invention to provide a single and a multi-cycle evacuation and unloading process and also to provide same in cycled combination with a pressurization and loading process for removing gases, whether entrained or absorbed, with or without compacting the structure.
It is an object of the invention to provide an improved evacuation process in combination with a pressurization process for fiber-reinforced composite manufacture, especially suitable for use on large structures outside an autoclave.
Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus and method are disclosed, in suitable detail to enable one of ordinary skill in the art to make and use the invention. In certain embodiments an apparatus and method in accordance with the present invention may include one, more than one, or all features of the invention. Four principal features include homogeneous-resin-stitched reinforcing fiber structures (preforms), resin-packet transfer molding to infuse resin into reinforcing fiber structures, multiple containment (bagging), and cyclical evacuation and pressurization of structures during manufacturing for ensuring minimum voids, maximum flow, optimum strength, and maximum structural integrity.
Thus, in certain embodiments, the invention may include a preform manufactured as if it were a layup or filament winding of a structure. However, only a small fraction of the rovings will actually be coated with resin during the layup. As a result, resined tows or rovings will bond, upon curing, to adjacent tows and rovings, and especially to other, sparsely located, resined tows. The result is a structurally stable (even rigid, if desired) reinforcement preform containing a comparatively small amount (typically less than 20 percent, to less than 5 percent) of its ultimate resin capacity.
In certain embodiments, an apparatus and method in accordance with the invention may provide a resin packet having a perforated sidewall. The resin therein may be contained as a gel, as a viscous or inviscid liquid, or as a thixatropic suspension. A stabilizing mat for preventing collapse of the packet or for rapid flow of the resin may fit inside, between two walls of a packet or envelope of resin. The resin may be selected to have a melting point or flow point above ambient conditions. Thus, the resin packet shape may be comparatively stable for handling. In one embodiment, the resin may have such a high viscosity as to have unnoticeable flow properties at ambient conditions.
Nevertheless, by whatever means, the resin may later be heated or otherwise rendered flowing for ready infusion into a preform. The resin may be drawn by an applied vacuum through a perforated wall of the envelope to flow directly and transversely through a preform or composite fiber layup.
In certain embodiments of apparatus and methods in accordance with the invention, multiple xe2x80x9cbagsxe2x80x9d (chambers) may provide cyclical control of evacuation of gases from a preform contained therein. In selected embodiments, vacuum bags may be doubled under a bell or a bell may be treated as a second vacuum chamber for both relieving and raising pressure in order to expedite evacuation of entrapped air and consolidation of parts.